1. Field of the Invention
The present invention relates to a technique for the simultaneous transmission of analog speech and modulated data over an analog channel with the capability at the receiver of separating the two simultaneously received signals and substantially improving the cancellation of the data from the speech by compensating for phase jitter and frequency offset in the recovered data signal.
2. Description of the Prior Art
Existing analog transmission facilities would be more efficient if speech and data could be simultaneously transmitted over the same channel. Preferably, such a proposal should not compromise the recovered speech and data qualities, neither should there be an expansion in the bandwidth requirements. At the same time, it is desirable to have a system which is simple and cost-effective.
A method of transmitting data and speech signals in a telephone system in which communication is effected via a radio link is disclosed in U.S. Pat. No. 4,280,020 issued to L. E. Schnurr on July 21, 1981. There the data and speech signals are separated in the frequency domain and transmitted in respective separate sideband channels, the data sideband channel containing sidebands generated by time coding an otherwise continuous wave signal.
A spread spectrum arrangement for (de)multiplexing speech signals and nonspeech signals was disclosed in U.S. Pat. No. 4,313,197 issued to N. F. Maxemchuk on Jan. 26, 1982. There, at the transmitter, a block of speech signals may be converted from the time domain to a frequency domain by a Fourier transformation. A Fourier component may be pseudo-randomly selected from a subset of such components. Responsive to the selected components, a prediction of the component may be substituted therefor, the prediction being thereafter modified, e.g., by its amplitude being incremented or decremented to reflect the multiplexing of a logic 1 or a logic 0 nonspeech signal. The modified prediction may be converted back to the time domain for transmission to the receiver. At the receiver, a parallel demultiplexing occurs for extracting speech signals and nonspeech signals for the multiplexed signals.
Recently several systems have been proposed to send speech and data simultaneously which exploit the properties of the Short Time Fast Fourier Transform (FFT) and the statistical properties of speech. For example, in the article "Simultaneous Transmission of Speech and Data using Code-Breaking Techniques" by R. Steele et al. in BSTJ, Vol. 60, No. 9, November 1981 at pages 2081-2105, a system whereby speech is used as a data carrier is proposed. More particularly, the speech, sampled at 8 kHz, is divided into blocks of N samples, and provided the correlation coefficient and mean square value of the samples exceed system thresholds, data is allowed to be transmitted. If the data is a logical 0, the samples are sent without modification; however, if a logical 1 is present, frequency inversion scrambling of the samples occurs. The receiver performs the inverse process to recover both the speech and data. These techniques can be quite complex and require careful timing and non-dispersive channels.
The problem remaining is to provide a technique for the simultaneous transmission of speech and data over an analog channel while compensating at the receiver for various effects produced by the analog channel which technique is simple and cost effective and does not require an expansion in bandwidth requirements.